1. Field of the Invention
The invention pertains to a power supply device for a thermionic emitting cathode.
2. Prior Art
Electron guns are used in numerous areas. One field of application is vaporization technology, in which solid materials, e.g. metals, are melted and then vaporized through the impact of electrons.
Several devices are known to generate electron beams. One of these devices is the so-called thermionic emitting cathode, which emits electrons by using the Edison effect. A metal filament heated by the passage of current either itself forms the directly heated thermionic emitting cathode or supplies its heat to the thermionic emitting cathode and to the thoriated tungsten cathode. Directly heated alkaline earth oxide cathodes are used if the heating capacity should be kept as low as possible. Power supply presents a special problem for directly heated thermionic emitting cathodes, which are used for electron vapor deposition plants. It should at least fulfill a number of requirements simultaneously and, for example, be high-voltage protected, short circuit-proof, dynamically controlled, suitable for continuous duty, small and light, as well as, low maintenance and easy to service.
A power supply for a thermionic emitting cathode has been described in DE-PS 29 28 301, but the design of the latter has not been described in detail. In this reference a pulsed transformer is provided, however, the latter serves as Wehnelt voltage control.
A phase-angle control with thyristors, a transductor, or a regulating transformer can be used to control the heater current for thermionic emitting cathodes generally located on the primary of a filament transformer which is monitored by a current transformer. These controllers have the disadvantage that they weigh too much and that they are oversized, besides, they show high heating losses. Low efficiency is the result.
If an electron beam vaporizer is fed by one high-voltage source only, monitoring the emission current is made relatively simple. Here, the anode of the high-voltage rectifier is grounded across a resistor. The voltage across the resistor is proportional to the emission current. It is a disadvantage that the measurement is not potential related.
However, if several electron beam vaporizers are connected to one high-voltage source, then the measuring method of a single current monitoring as described above is not possible. An added total current of all electron beam vaporizers builds up at the measuring reference resistor. In such a case the emission current must be monitored with dc converters at outputs with high-voltage potential. This has the advantage of being galvanically decoupled. However, opposing this advantage is the high technical effort with mostly inaccurate detection of emission current.
Monitoring of short circuits is of great importance for power supply devices of thermionic emitting cathodes because during a short circuit the dynamic control of the emission current must be interupted immediately to prevent arcing. As a rule, the monitoring of short circuits is carried out by a resistor on a high-voltage generator, which also converts the dynamic increase of the short circuit current into a proportional voltage signal that will be analyzed afterwards. Here, too, the disadvantage is the potential related monitoring.